Photodynamic therapy (PDT, also known as photochemotherapy) is a therapeutic technique that uses a photosensitizing agent and a particular light source to activate the photosensitizer. Upon exposure to visible light, the photosensitizer is activated to high energy states and further reacts with oxygen or transfer its energy to oxygen molecules to create reactive oxygen species (ROS), such as singlet oxygen or free oxygen radicals. ROS are highly cytotoxic leading to apoptosis or necrosis of malignant and abnormal cells. The activated photosensitizer can also return to a lower energy level partly by emitting red fluorescence, which can be used for diagnostic purposes. PDT has a wide range of clinical applications in fields such as dermatology, diagnosis and cosmetic treatment.
Among the most commonly used photosensitizers or precursors in PDT are 5-aminolevulinic acid (5-ALA) and its two esters, 5-methylaminolevulinate (5-MAL) and 5-hexylaminolevulinate (5-HAL). Most cells of the human body can absorb and transform 5-ALA or its esters to photoactivatable porphyrins in particular protoporphyrin IX (PpIX), which is a very potent photosensitizer, via the porphyrin biosynthetic pathway. Studies have confirmed that cancerous, precancerous or diseased cells tend to accumulate more porphyrins than normal cells. This selectivity has been contributed to the dysfunction of abnormal cells or tissue, including reduced ferrochelatase in neoplasms (Kemmner et al, FASEB J, 2008, 22:500-509) or enhanced permeability and retention effects (Avci et al, J Biomed Nanottechnl, 2014, 10(9)1937-1952).
PDT using 5-ALA and its esters has proven to be a safe and effective therapeutic strategy with excellent cosmetic outcomes (Fonda-Pascual et al, Methods, 2016, 109, 190-202) in the treatment of certain cancers, pre-cancerous disorders, non-cancerous lesion and in cancer diagnoses (Jichlinski et al, J Urol, 2003, 170: 226-9). Since these agents are unstable and prone to polymerization, they are provided as hydrochloride salts in clinical applications. Levulan® (20% 5-aminolevulinic acid hydrochloride, DUSA Pharmaceuticals, Wilmington, Mass., USA) is approved for the treatment of actinic keratosis in US and ALA (20% 5-ALA hydrochloride, Fudan-Zhangjiang Bio-Pharma, Shanghai, China) for the treatment of external condyloma acuminatum in China. Metvix® (16% 5-MAL hydrochloride, PhotoCure ASA, Oslo, Norway) is used for the treatment of actinic keratosis, superficial basal cell carcinoma and Bowen's disease. Hexvix® (5-HAL hydrochloride, PhotoCure ASA, Oslo, Norway) is a highly potent photosensitizer for the diagnoses of bladder cancer.
5-ALA and 5-MAL are also currently in multiple clinical trials and in off-label use for the treatment of moderate to severe acne vulgaris (Mavilia et al, Br J Dematol, 2007, 157:779-846). In addition, preclinical and clinical PDT studies have shown that 5-ALA and its esters are effective to treat bacterial (Fotinos et al, Antimicrob Agents Chemother, 2008, 52:1366-1373) and fungal infections (Calzavara-Pinton et al, Photochem Photobiol, 2012, 88:512-522), inflammatory disorders (e.g. morphea, actinic cheiliti, acne) and infections associated with virus and cancers (e.g. viral warts).
Jeffry et al describe 5-ALA and its methyl ester as useful agents for photodynamic cosmetic treatment (Arch Dermatol, 2005 141:1247-1252). Carrasco et al demonstrate the ability of in situ ROS induced by 5-MAL-PDT to activate cell proliferation in mouse skin and in the hair follicle stem cell niche, promoting hair growth, tissue repair and wound healing (J Invest Dermatol, 2015, 135(11):2611-2622). All the references described herein are incorporated by reference.
However, hydrochloride salts of 5-ALA and its esters possess a number of undesirable properties, including skin irritation, inflammation and pain at the treatment site upon light exposure (Grapengiesser et al, Clin Exp Dermatol, 2002, 27:493-497), hygroscopic which may cause degradation during storage and instability in pharmaceutical formulations (Gadmar et al, J. Photochem. Photobiol B: Biol, 2002, 67:187-193). The other major limitation of topical 5-ALA and 5-MAL-PDT is the depth of irradiation and skin penetration of the drug. Inadequate skin penetration was considered to play a major role. Therefore, enhancing skin penetration of these agents would improve PDT effectiveness (Foster et al, Exp Dermatol, 2010, 19:806-12).
U.S. Pat. No. 8,692,014 describes that sulfonate salts of 5-ALA analogs exhibit reduced hydroscopicity and enhanced fluorescence production in cells in comparison with hydrochloride salts. U.S. Pat. No. 8,173,839 describes that 5-ALA phosphate salt is less stimulating/irritating to tongue surface of the body than 5-ALA hydrochloride.
Trichloroacetic acid (TCA) is a small organic acid with a molecular weight greater than acetic acid and hydrochloric acid. It is a potent chemical cauterant and widely used as topical medication and in cosmetic treatment. The acid has proven ability to penetrate deeper layers of skin, coagulate epidermal protein, to destruct and remove damaged skin cells. TCA also exhibits anti-bacterial and anti-inflammatory activities, and further it can stimulate cell division in the basal skin layer to form new cells and uniformly distribute natural skin pigment melanin. Therapeutic outcomes of TCA are often dependent to the depth of its penetration, which is directly proportional to its concentrations applied. Low concentrations of TCA (10-20%) are used as superficial peels or epidermal peels, which penetrate and affect the depth of the epidermis and the interface of the dermis-epidermis. This concentration range is used to treat mild photoaging, melisma, comedonal acne and postinflammatory erythema. Medium-depth peels or dermal peels of TCA (20-50%), extending the penetration through the epidermis to the papillary dermis, are used for the treatment of moderate photoaging, actinic keratosis and mild acne scarring (Di et al, Photodermatol Photoimmunol Photomed. 2015, 233-8). At 50% or higher, deep peels penetrate midreticular dermis to treat external genital wart and acne scar (Yanofsky et al, Expert Rev Dermatol, 2013, 8:321-332; Agarwal et al, Dermatol Surg, 2015, 41:597-604).
Both TCA peeling and 5-ALA or 5-MAL PDT are widely used in dermatology and cosmetic care but often show different therapeutic profiles. For example, in the treatment of acne vulgaris, comedonal acne/noninflammatory lesions respond well to TCA peels, whereas papulopustular acne/inflammatory lesions show moderate improvement (Meguid et al, Dermatol Surg, 2015, 41:1398-1404). In contrast, 5-ALA PDT produces significant improvements in papulopustular acne/inflammatory lesions, but poor responses in comedonal/noninflammatory lesions. The different outcomes may be explained by their mechanism of actions. 5-ALA PDT selectively destructs the sebaceous unit (Wan et al, Clin Cosmed Investig Dermatol, 2014, 7:145-163). The mechanism of TCA in the treatment of acne vulgaris is believed to be due to its ability to diminish corneocyte cohesion and keratinocyte plugging. Therefore, a salt of TCA with 5-ALA or its esters may provide synergistic effects or dual activities in treating acne vulgaris and other diseases or disorders described herein. The present inventors have found that the salts of TCA with 5-ALA or its esters are less irritating/stimulating to the sensitive surface of the body when compared with the hydrochloride salts. The present inventors have also found that TCA salts of 5-MAL and 5-HAL are resistant to moisture and much less hygroscopic than corresponding hydrochloride salts.